A disk drive is a data storage device that stores data in concentric tracks on a disk shaped medium (i.e., a disk). A disk drive is generally used as a mass storage device for an external host computer that is connected to the disk drive via an input/output port. During normal disk drive operation, the host computer delivers an access request (i.e., a read request or a write request) to the disk drive requesting a transfer of customer data between the host computer and the disk. The disk drive then performs the requested data transfer by performing either a read operation (where data is transferred from the disk to the host computer) or a write operation (where data is transferred from the host computer to the disk). In the access request, the host computer generally indicates a location on the disk surface from/to which the customer data is to be transferred. For example, the host computer can specify a target track and sector on the disk or it can indicate a logical block address from which a target track and sector can be calculated. The disk drive then moves a transducer to a position above the identified disk location and the transfer of customer data is initiated.
In addition to customer data, the surface of the disk in a disk drive normally includes some non-customer data (i.e., overhead data) to facilitate proper disk drive operation. For example, the disk surface usually includes servo data for use by the disk drive to position the transducer during read and write operations. The servo data can include among other things: coarse position information (e.g., track address), and fine position information (e.g., servo bursts) for use in determining transducer position, and correction information (e.g., embedded runout correction (ERC) values) for use in compensating for rotational peculiarities in the disk structure. During disk drive operation, the transducer periodically reads the servo data from the surface of the disk. The disk drive then uses the servo data to appropriately move the transducer to the desired position.
As can be appreciated, proper disk drive operation depends on the accurate and reliable reproduction of the servo data written on the disk. Servo data has traditionally been written in radially aligned servo spokes occurring at equal intervals about the circumference of the disk. The servo spokes are generally written during disk drive manufacture by a highly accurate and very expensive piece of equipment known as a servo track writer (STW). Accordingly, the servo spokes cannot be modified by the customer after disk drive delivery (i.e., they are read-only). In addition, the information within the servo spokes is written at a different (i.e., lower) frequency from the other data on the disk surface. Servo spokes have traditionally included a minimal level of error detection capability, usually comprising a single parity bit. In contrast, customer data regions have generally used very sophisticated error detection/correction schemes to insure data integrity.
An ongoing trend in the disk drive industry is to store an ever increasing amount of data within a given area on a disk surface (i.e., to increase disk data densities). Along with this increase in data density comes an increase in the difficulty with which servo data can be accurately reproduced and utilized. Thus, current methods for storing servo information on the disk surface are rapidly becoming inadequate. In addition, current methods are generally expensive to implement (e.g., STW costs) and do not permit servo re-calibrations to be performed in the field. Current methods also make inefficient use of disk space as all servo information is generally written at a lower frequency and thus takes up a greater amount of disk surface area.
Therefore, there is need in the disk drive industry for new techniques for arranging and using servo data in a disk drive. The techniques will preferably be highly reliable for use in high capacity disk drives and should be relatively inexpensive to implement. In addition, the techniques will preferably allow servo re-calibrations to be performed in the field. Furthermore, it is desirable that the techniques make relatively efficient use of disk surface space.